A method includes forming a first and a second contact opening to reveal a first and a second source/drain region, respectively, forming a mask layer having a first and a second portion in the first and the second contact openings, respectively, forming a first and a second sacrificial ILD in the first and the second contact openings, respectively, removing the first sacrificial ILD from the first contact opening, filling a filler in the first contact opening, and etching the second sacrificial ILD. The filler protects the first portion of the mask layer from being etched. An ILD is formed in the second contact opening and on the second portion of the mask layer. The filler and the first portion of the mask layer are removed using a wet etch to reveal the first contact opening. A contact plug is formed in the first contact opening.